ABSTRACT

In-situ burning is recognised as a viable alternative for cleaning up oil spills on land and water. When performed under optimal conditions, in-situ burning can rapidly reduce the volume of spilled oil and eliminate the need to collect, store, transport and dispose of recovered oil. In-situ burning can shorten the response time to an oil spill, thus reducing the chances that the oil will spread on the water surface or penetrate further into land and thereby aiding in environmental protection. This chapter contains a compilation of information about in-situ burning of oil spills on water. It includes the scientific aspects of the burning process and its effects, and practical information about the procedures to be followed and equipment required for carrying out an in-situ burn on water.

Ignition is easy for volatile oils and is more difficult for heavier oils which require a primer such as diesel fuel for sufficient heat. If insufficient vapours are produced, the fire will either not start or will be quickly extinguished. The quantity of vapours produced depends on the amount of heat radiated back to the oil. If the oil slick is too thin, some of this heat is conducted to the water layer below it. Oil that is completely emulsified with water can be ignited, given that sufficient heat is supplied, typically by burning it alongside un-emulsified oil. Containment of the oil on water may be necessary to carry out in-situ burning as the oil must be thick enough to quantitatively burn. Once burning, the heat radiated back to the slick and the insulation are usually sufficient to allow combustion down to about 0.5 to 1 mm of oil. The oil burn rate is a largely a function of oil type and environmental conditions.

The residue from burning oil is largely unburned oil with some lighter or more volatile products removed. When the fire ceases, unburned oil is left that is simply too thin to sustain combustion. In addition to unburned oil, weathered oil is present that has been subjected to high heat. Heavier soot particles are re-precipitated from the smoke plume into the fire and thus become part of the residue. Highly efficient burns of some types of heavy crude oil may result in oil residue that sinks after cooling in sea water.

Safety is of prime concern. A burn plan and a safety plan must be prepared to encompass the concerns noted in this chapter. Training of personnel, including field practice, is essential to ensure safety.

The following emissions of burning can be of concern:

Particulate matter/soot: Burns produce an abundance of particulate matter (soot). Particulate matter at ground level is a health concern close to the fire and under the plume. The particulate concentrations from in-situ oil fires should be monitored for some burns.

Polyaromatic hydrocarbons (PAHs): Oils contain significant quantities of PAHs which are largely destroyed in combustion. The PAH concentrations in the smoke (soot) in the plume and in the particulate precipitation at ground level are much lower than those in the starting oil.

Volatile organic compounds (VOCs): Many VOCs are emitted by fires, but in lesser quantity than when the oil is not burning. VOCs are not a significant concern.

Organic compounds: No exotic or highly toxic compounds are generated as a result of the combustion process. Organic macro-molecules are in lesser concentration in the smoke and downwind than they are in the oil itself. Dioxins and dibenzofurans have not been measured as emissions of oil fires.

Carbonyls: Carbonyls such as aldehydes and ketones are created by oil fires, but they do not exceed health concern levels even close to fires.

Gases: Combustion gases such as carbon dioxide, carbon monoxide and sulphur dioxide are produced by oil fires but are significantly below any health concern level.

Overall, emissions are now understood to the extent that emission levels and safe distances can be predicted for fires of various sizes and types.